b2sum: Add digest length argument

It may be desirable to produce a digest with a shorter length than the default
using a BLAKE2 algorithm. For example, the crypto_generichash() function in
libsodium uses BLAKE2b but shortens the digest to 256 bits by default and it is
useful for debugging to generate similar hashes with b2sum for comparison.

The requested digest length is specified in bits, must be a multiple of 8 and
cannot exceed the total output length of the selected algoritm. No minimum
digest length is enforced.

If a shorter-than-default digest length is selected, this is indicated in the
output when the '--tag' option is given.

1 files changed, 49 insertions, 22 deletions

diff --git a/b2sum/b2sum.c b/b2sum/b2sum.c
index 36ecd52..af01a81 100644
--- a/b2sum/b2sum.c
+++ b/b2sum/b2sum.c
@@ -25,7 +25,7 @@
 #include "blake2.h"
 
 /* This will help compatibility with coreutils */
-int blake2s_stream( FILE *stream, void *resstream )
+int blake2s_stream( FILE *stream, void *resstream, size_t outbytes )
 {
   int ret = -1;
   size_t sum, n;
@@ -35,7 +35,7 @@ int blake2s_stream( FILE *stream, void *resstream )
 
   if( !buffer ) return -1;
 
-  blake2s_init( S, BLAKE2S_OUTBYTES );
+  blake2s_init( S, outbytes );
 
   while( 1 )
   {
@@ -68,14 +68,14 @@ final_process:;
 
   if( sum > 0 ) blake2s_update( S, buffer, sum );
 
-  blake2s_final( S, resstream, BLAKE2S_OUTBYTES );
+  blake2s_final( S, resstream, outbytes );
   ret = 0;
 cleanup_buffer:
   free( buffer );
   return ret;
 }
 
-int blake2b_stream( FILE *stream, void *resstream )
+int blake2b_stream( FILE *stream, void *resstream, size_t outbytes )
 {
   int ret = -1;
   size_t sum, n;
@@ -85,7 +85,7 @@ int blake2b_stream( FILE *stream, void *resstream )
 
   if( !buffer ) return -1;
 
-  blake2b_init( S, BLAKE2B_OUTBYTES );
+  blake2b_init( S, outbytes );
 
   while( 1 )
   {
@@ -118,14 +118,14 @@ final_process:;
 
   if( sum > 0 ) blake2b_update( S, buffer, sum );
 
-  blake2b_final( S, resstream, BLAKE2B_OUTBYTES );
+  blake2b_final( S, resstream, outbytes );
   ret = 0;
 cleanup_buffer:
   free( buffer );
   return ret;
 }
 
-int blake2sp_stream( FILE *stream, void *resstream )
+int blake2sp_stream( FILE *stream, void *resstream, size_t outbytes )
 {
   int ret = -1;
   size_t sum, n;
@@ -135,7 +135,7 @@ int blake2sp_stream( FILE *stream, void *resstream )
 
   if( !buffer ) return -1;
 
-  blake2sp_init( S, BLAKE2S_OUTBYTES );
+  blake2sp_init( S, outbytes );
 
   while( 1 )
   {
@@ -168,7 +168,7 @@ final_process:;
 
   if( sum > 0 ) blake2sp_update( S, buffer, sum );
 
-  blake2sp_final( S, resstream, BLAKE2S_OUTBYTES );
+  blake2sp_final( S, resstream, outbytes );
   ret = 0;
 cleanup_buffer:
   free( buffer );
@@ -176,7 +176,7 @@ cleanup_buffer:
 }
 
 
-int blake2bp_stream( FILE *stream, void *resstream )
+int blake2bp_stream( FILE *stream, void *resstream, size_t outbytes )
 {
   int ret = -1;
   size_t sum, n;
@@ -186,7 +186,7 @@ int blake2bp_stream( FILE *stream, void *resstream )
 
   if( !buffer ) return -1;
 
-  blake2bp_init( S, BLAKE2B_OUTBYTES );
+  blake2bp_init( S, outbytes );
 
   while( 1 )
   {
@@ -219,14 +219,14 @@ final_process:;
 
   if( sum > 0 ) blake2bp_update( S, buffer, sum );
 
-  blake2bp_final( S, resstream, BLAKE2B_OUTBYTES );
+  blake2bp_final( S, resstream, outbytes );
   ret = 0;
 cleanup_buffer:
   free( buffer );
   return ret;
 }
 
-typedef int ( *blake2fn )( FILE *, void * );
+typedef int ( *blake2fn )( FILE *, void *, size_t );
 
 
 static void usage( char **argv, int errcode )
@@ -238,6 +238,8 @@ static void usage( char **argv, int errcode )
   fprintf( out, "\n" );
   fprintf( out, "  -a <algo>    hash algorithm (blake2b is default): \n"
                 "               [blake2b|blake2s|blake2bp|blake2sp]\n" );
+  fprintf( out, "  -l <length>  digest length in bits, must not exceed the maximum for\n"
+                "               the selected algorithm and must be a multiple of 8\n" );
   fprintf( out, "  --tag        create a BSD-style checksum\n" );
   fprintf( out, "  --help       display this help and exit\n" );
   exit( errcode );
@@ -247,8 +249,9 @@ static void usage( char **argv, int errcode )
 int main( int argc, char **argv )
 {
   blake2fn blake2_stream = blake2b_stream;
-  size_t outlen   = BLAKE2B_OUTBYTES;
+  size_t maxbytes = BLAKE2B_OUTBYTES;
   const char *algorithm = "BLAKE2b";
+  size_t outbytes = 0;
   unsigned char hash[BLAKE2B_OUTBYTES] = {0};
   bool bsdstyle = false;
   int c;
@@ -258,13 +261,15 @@ int main( int argc, char **argv )
   {
     int this_option_optind = optind ? optind : 1;
     int option_index = 0;
+    char *end = NULL;
+    size_t outbits;
     static struct option long_options[] = {
       { "help",  no_argument, 0,  0  },
       { "tag",   no_argument, 0,  0  },
       { NULL, 0, NULL, 0 }
     };
 
-    c = getopt_long( argc, argv, "a:", long_options, &option_index );
+    c = getopt_long( argc, argv, "a:l:", long_options, &option_index );
     if( c == -1 ) break;
     switch( c )
     {
@@ -272,25 +277,25 @@ int main( int argc, char **argv )
       if( 0 == strcmp( optarg, "blake2b" ) )
       {
         blake2_stream = blake2b_stream;
-        outlen = BLAKE2B_OUTBYTES;
+        maxbytes = BLAKE2B_OUTBYTES;
         algorithm = "BLAKE2b";
       }
       else if ( 0 == strcmp( optarg, "blake2s" ) )
       {
         blake2_stream = blake2s_stream;
-        outlen = BLAKE2S_OUTBYTES;
+        maxbytes = BLAKE2S_OUTBYTES;
         algorithm = "BLAKE2s";
       }
       else if ( 0 == strcmp( optarg, "blake2bp" ) )
       {
         blake2_stream = blake2bp_stream;
-        outlen = BLAKE2B_OUTBYTES;
+        maxbytes = BLAKE2B_OUTBYTES;
         algorithm = "BLAKE2bp";
       }
       else if ( 0 == strcmp( optarg, "blake2sp" ) )
       {
         blake2_stream = blake2sp_stream;
-        outlen = BLAKE2S_OUTBYTES;
+        maxbytes = BLAKE2S_OUTBYTES;
         algorithm = "BLAKE2sp";
       }
       else
@@ -301,6 +306,16 @@ int main( int argc, char **argv )
 
       break;
 
+    case 'l':
+      outbits = strtoul(optarg, &end, 10);
+      if( !end || *end != '\0' || outbits % 8 )
+      {
+        printf( "Invalid length argument: `%s'\n", optarg);
+        usage( argv, 111 );
+      }
+      outbytes = outbits / 8;
+      break;
+
     case 0:
       if( 0 == strcmp( "help", long_options[option_index].name ) )
         usage( argv, 0 );
@@ -314,6 +329,15 @@ int main( int argc, char **argv )
     }
   }
 
+  if(outbytes > maxbytes)
+  {
+    printf( "Invalid length argument: %zu\n", outbytes * 8 );
+    printf( "Maximum digest length for %s is %zu\n", algorithm, maxbytes * 8 );
+    usage( argv, 111 );
+  }
+  else if( outbytes == 0 )
+    outbytes = maxbytes;
+
   if( optind == argc )
     argv[argc++] = (char *) "-";
 
@@ -331,7 +355,7 @@ int main( int argc, char **argv )
       continue;
     }
 
-    if( blake2_stream( f, hash ) < 0 )
+    if( blake2_stream( f, hash, outbytes ) < 0 )
     {
       fprintf( stderr, "Failed to hash `%s'\n", argv[i] );
     }
@@ -339,10 +363,13 @@ int main( int argc, char **argv )
     {
       if( bsdstyle )
       {
-        printf( "%s (%s) = ", algorithm, argv[i] );
+        if( outbytes < maxbytes )
+          printf( "%s-%zu (%s) = ", algorithm, outbytes * 8, argv[i] );
+        else
+          printf( "%s (%s) = ", algorithm, argv[i] );
       }
 
-      for( size_t j = 0; j < outlen; ++j )
+      for( size_t j = 0; j < outbytes; ++j )
         printf( "%02x", hash[j] );
 
       if( bsdstyle )